Abstract
The spectroscopic behaviour of lumiflavin (7,8,10-trimethyl-isoalloxazine, oxidized form LF ox) in aqueous solutions of pH range −1.08 to 14.6 is studied. Absorption spectra, fluorescence quantum distributions, quantum yields and lifetimes are determined. The ionization stage of ground-state LF ox changes from cationic (LF oxH 2 +) at low pH (p K c ≈ 0.38) via neutral (LF oxH) to anionic (LF ox −) at high pH (p K a ≈ 10.8). The cationic, neutral, and anionic forms are identified by their different absorption spectra. LF oxH in neutral aqueous solution is reasonably fluorescent (fluorescence quantum yield ϕ F ≈ 0.29, fluorescence lifetime τ F ≈ 5.2 ns), while LF ox − is weakly fluorescent ( ϕ F ≈ 0.0042, τ F ≈ 90 ps), and LF oxH 2 + is nearly non-fluorescent ( ϕ F ≈ 3.6 × 10 −5, τ F ≈ 0.4 ps). A theory of the pH dependent equilibration of cationic, neutral and anionic molecules in the ground state and their dynamics in the excited state is developed. For lumiflavin in aqueous solution in the excited state no equilibrium distributions are reached between the cationic, neutral, and anionic forms. Some neutral excited lumiflavin transforms to the cationic ground-state form at low pH by intermolecular photo-induced proton transfer from H 3O + to LF oxH*. At high pH no photo-induced intermolecular proton transfer takes place.
Published Version
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